Transmitting system



Fell 1935- D. E. O'DONOVAN ET AL I 1,991,027

TRANSMITTING SYSTEM Filed May 11, 1932 INYENTORS I DANIEL EDWARD ODONOVAN BYLESLIE fig? NIXON wz/M/ ATTORNEY Patented Feb. 12, 1935 UNITEDSTATES PATENT OFFICE TRANSMITTING SYSTEM ware Application May 11, 1932,Serial No. 610,526 In Great Britain May 15, 1931 4 Claims.

This invention relates to radio and other high frequency transmittingsystems and more particularly to keying systems of the kind employingso-called absorber and sub-absorber valves. The

invention has for its object to remove certain disadvantages met with inknown transmitters employing keying systems of the absorber andsubabsorber type.

Keying systems of the absorber and subabsorber type are well known andconsist broadly of arrangements wherein signaling is accomplished bycontrollably diverting anode feed from a stage of a transmitter by meansof a valve termed an absorber valve whose grid is indirectly controlledso that the said valve becomes conductive and non-conductive independence upon the signals, the control of grid potential for theabsorber valve being in turn effected by a sub-absorber valve whose gridis directly controlled, for example, by a key, so that the saidsubabsorber valve becomes conductive and non-conductive, the consequentvariations in plate potential of the sub-absorber valve forming part ofthe potential applied to the grid of the absorber valve. Thus when thekey or equivalent device is operated, thetransmitter is keyed by varyingthe voltage available at the terminals of the transmitter stageassociated with the absorber valve. Such keying systems ofier theadvantages, inter alia, that the value of current or a portion of thevalue of current taken by the transmitter during signals is divertedthrough the absorber valve at other times, while, owing to the fact thatthe absorber valve is in turn controlled by the subabsorber valve, theload or duty upon the keying means may be made quite small.

The present invention relates to keying circuits and systems of theabsorber type, as briefly described above, and involves improvementsover said systems as known heretofore in the art. The nature of saidimprovements will be better understood from the following detaileddescription thereof when read in connection with the drawing, throughoutwhich like reference symbols indicate like parts, and in which:

Figure 1 shows a keying system including an oscillation generator, anabsorber tube, and a sub-absorber tube; 7 while,

Figures 2 and 3 show circuits of the type disclosed in Figure 1 whichinclude novel features and improvements.

The circuit of Figure 1 comprises an absorber valve AV whoseanode-cathode space is shunted across any one of the tubes T in anystage of the transmitter to be controlled. The tube T has been shownconnected in an oscillation generator circuit which is coupled to a workcircuit W. C. This work circuit may be an aerial system. The anode ofthe said absorber valve being connected through an absorber resistanceAR to 5 the positive terminals of the anode supply and the cathode beingdirectly connected to the negative terminal of the supply 10. Alsoshunted between the positive and negative terminals of the anode supplyis a sub-absorber resistance 10 SAR in series with the anode-cathodespace impedance of a sub-absorber valve SAV, the oathode oi the saidvalve being connected to the negative terminal of the anode supply. Theanode of the sub-absorber valve is connected through a suitable sourceof negative bias potential NB to the grid of the absorber valve andmeans, such as a key K associated with suitable sources of potential SPare provided for varying the potential between grid and cathode of thesub-absorber valve. It will be seen that with this arrangement, if thegrid of the sub-absorber valve is given a suitable negative potential bymoving the key into one position little or no anode current will passthrough the sub-absorber valve, and in consequence the grid of theabsorber valve will become positive. Owing to the grid current taken bythe absorber valve a fall of potential will occur down the resistanceSAR in the anode circuit of the sub-absorber valve, this wall ofpotential causing the grid of the absorber valve to assume a positivepotential considerably less than the difference between the anode supplypotential and the bias potential applied to the grid of the absorbervalve. If now, the sub-absorber valve grid is given a suitable positivepotential, by moving the key to its other position the subabsorber valvewill draw current through the sub-absorber resistance, the potential orthe anode of the sub-absorber valve will fall and the potential of theabsorber grid will become negative to a degree determined by the sourceof bias potential so that the said absorber valve (which, in the otherposition of the key, takes substantial current from the anode supply andthus reduces the voltage upon the transmitter stage being controlled)becomes non-conductive, so that the full or substantially the full anodesupply voltage is available for the transmitter stage. Thus in the caseof a telegraph transmitter operating in the usual way between markingand spacing the transmitter will be on mark when the subabsorber grid ispositive and on space when the sub-absorber grid is negative.

An inherent disadvantage of a circuit such as 6| filament itself shouldfail, the sub-absorber resistance SAR, the bias battery NB, the absorbervalve grid, and thesub-absorber valve anode, will all assume thepotential of the positive terminal of the anode supply since no gridcurrent will flow in the absorber valve.

This, of course, involves the necessity of insulating the parts namedfor the full potential of the anode supply, and moreover, since thesub-absorber valve is generally de- 7 signed for low voltages only, thisvalve is likely to be destroyed should the filament orfilament supply ofthe absorber valve fail as supposed. In

addition, owing to the high potential which the grid of the absorbervalve would attain in these circumstances, the said absorber valve islikely'to be damaged. A known method of overcoming this difficultyisshown in Figure 2 and consists in employing in eifect a potentiometerresistance between 'the'sub-absorber valve anode, and a suitable sourceof negative potential and in one arrangement of this kind the absorberand subabsorber keying circuit shown in Figure 1 is modified byreplacingthe source of bias potential NB between the anode of the sub-absorbervalve'and the grid of the absorbervalve by a resistance RB andconnecting the grid of the absorber valve through a further resistanceGR to a suitable source of negativepotential GNB, the value of theresistances in the circuit being so chosen that in the event of failureof filament supply to the absorber valve, or in the event of filamentfailure in the absorber valve, the potentials obtained are notdangerous. This circuit though obviating the disadvantages above setforth, offers other disadvantages, the chief of which are that theresistive circuit consisting of the sub-absorber resistance SAR, theresistance RB between theanode of'the sub-absorber valve and the grid ofthe absorber valve and the resistance GR between the grid ofthe'absorber valve and the source of negative potential, causes currentto be taken from the high tension supply in excess of thatrequiredmerely to provide grid current for the absorber valve, while, since thesub-absorber valve has to deal with a larger anode current than in thecase in which the potentiometer arrangement is not employed, this valvehas either to be a larger valve or must consist of a number of valves inparallel. This potentiometer arrangement, therefore, offers thedisadvantage of being practically uneconomical. r

The present invention has for its object to provide an absorber andsub-absorber keying system in which the dangers above referred to areavoided without lack of economy as compared to the first describedabsorber and sub-absorber circuit.

According to thisinvention, in an absorber and sub-absorber keyingsystem there is provided across the terminals of the source of anodepotential 10 a shunt path having connection to the grid of the absorbervalve, said shunt path includi'ng'a device ofiering an approximatelyinfinite resistance or a very high resistance during normal'operationsof the absorber and sub-absorber valves, but being adapted to offer areduced reno current during normal operation.

rectifier R (e. g. of the well known cuprous oxide type) between thegrid of the absorber valve and a suitable source of biasing potentialABP arranged to be slightly more positive than the potential attained bythe absorber grid in, normal service; It will be seen that under normalconditicns, therefore, the rectifier will pass substantially no current,butshouldrfor any reason, the

- absorber grid attain an unduly high potential, the

rectifier will become conductive and provide a path between the grid ofAV and ground and'thus prevent a dangerous rise of potential on theelements connected with the positive terminal of direct current source10. It will be seen that in this circuit arrangement the shunt pathcomprises the sub-absorber valve resistance SAR the source of biaspotential NB the rectifier R and the source of positive potential ABP towhich one end of the rectifier is connected.

In a modification (not illustrated) the rectifier Ris replaced by one ormore glow discharge tubes, such for instance as neon lamps. In this casethe potential of the bias source ABP to which the side of the lamp orlamp combination remote from the absorber valve grid is connected,should be of such value that Lmder normal service conditions thepotential drop across the lamp or lamps is below the ignition potential.If, however, the absorber valve grid rises in potential by apredetermined amount, the lamp or lamps will light and provide acomparatively low resistance path.

In a modification of the last described arrangement'the discharge lampor lamps is or are re placed by one or more spark gaps.

In arrangements in accordance-with this invention the bias potentialwithwhich the rectifier or other safety resistance device is associatedmay conveniently be obtained from the grid negative and positive supplyfrom which the keying potentials for the sub-absorber grid are obtained,although for the sake of simplicity in the drawing the various sourcesof potential are represented as constituted by separate batteries.

What we claim, is:

1. In a signalling system, a thermionic tube having electrodes includinganode, cathode, and control grid, and coupled circuits connected betweensaid electrodes to produceoscillations in said tube and circuits, meansfor-interrupting the oscillations produced in said tube and circuitscomprising, a second thermionic tube having anode, cathodeand controlelectrode, a'circuit for connecting the anode and cathode of said secondnamed tube in parallel with the anode and cathode of said first'namedtube, a circuit for connecting a source of potential in parallel withthe anode and cathode of both of said tubes, a keying device connectedin parallel with the control grid and cathode of said second named tube,and a source of potential and a rectifier connected between the controlgrid and cathode of said second named tube, said source of potentialbeing of a value greater than the normal operating potential appliedduring keying between the grid and cathode of said last named tube, saidrectifier being conductive when excess potentials areapplied between thegrid and cathode of said last named tube during keying. V

2. In a signalling system, a thermionic tube having an anode, a controlgrid and a cathode, oscillation producing circuits connected between theelectrodes of said tube, means for keying the oscillations produced insaid tube and circuit comprising, a thermionic tube having anode,cathode and control grids, a circuit connecting the anode and cathode ofsaid last named tube in parallel with the anode and cathode of saidfirst named tube, a third thermionic tube having anode, cathode andcontrol grid, a source of potential connecting the anode of said thirdnamed tube to the control grid of said second named tube, a connectionbetween the cathodes of all of said tubes, a rectifier connected betweenthe control grid and the cathode of said second named tube, saidrectifier being non-conductive at the normal potential applied to saidcontrol grid during operation, a key connected with the control grid ofsaid third named tube, and a source of potential having its negativeterminal connected to the cathodes of all of said tubes and its positiveterminal connected by way of resistances to the anodes of all of saidtubes.

3. In a signalling system, an oscillation generator including athermionic tube having a control grid an anode and a cathode, andoscillation circuits connected between said electrodes of said tube,keying means connected with said tube comprising, a second thermionictube having its anode to cathode impedance connected between the anodeand cathode of said first named tube, keying means connected between thecontrol grid and cathode of said last named tube, and a source ofpotential and a rectifier connected in parallel with said keying means,said source of potential being of a value in excess of the normaloperating potential applied during keying between the grid and cathodeof said last named tube, said rectifier being conductive when excesspotentials are applied between the grid and cathode of said last namedtube during operation.

4. A device for producing potentials, the amplitude of which may bevaried at signal irequency comprising, a thermionic tube having anode,cathode and control grid, a source of direct current potential,resistive means connecting the anode of said tube to the positiveterminal of said source of direct current potential, a connectionbetween the cathode of said tube and the other terminal of said source,a resistance and a source of potential connected between the controlgrid of said tube and the positive terminal of said source, a circuitfor applying signalling potentials between said resistance and thecathode of said tube, and means for limiting the value of the potentialswhich may be applied to said control grid during operation comprising adevice conductive in one direction only connected between the controlgrid and cathode of said tube.

DANIEL EDWARD ODONOVAN. LESLIE REGINALD NIXON.

